Prior to the development of the present invention, as is generally quite well known in the wheel slip correction art, a three state dump valve was being used in state of the art wheel slip correction arrangements installed on passenger transit type railway vehicles. The three states of these three state dump valves include: an application state, a release state and a lap state. Such lap state, as is known in the wheel slip correction art, creates a hold state.
These three state dump valves are relatively expensive and for this reason their acceptance for use in this application may not be as widespread as one would normally expect it to be. Such three state dump valves are, at present, commercially available in either a single coil three state dump valve or as a two coil three state dump valve. In any case, however, regardless of the three state dump valve being used, they are both technically quite complex. As would generally be expected, the technical complexity of these three state dump valves is believed to be a still further detriment to their having achieved widespread acceptance. Additionally, such three state dump valves are relatively heavy and, consequently, add to the overall weight of the wheel slip correction system and thus require the use of additional energy in order to move the vehicle having them installed thereon.
Nevertheless, because of the significant advantages to be gained by the use of such wheel slip correction arrangements on passenger transit type railway vehicles, these three state dump valves are still being used rather extensively.